A charged particle controller used in a mass spectrometer uses a radiofrequency field that is spatially and temporally modulated to control the movement of ions (see Patent Literature 1).
For example, in an ion guide that includes n rod electrodes (n is an even number) arranged so as to surround an ion beam axis, a radio-frequency voltage VRF having an amplitude of V and a frequency of Ω is applied so that adjacent rod electrodes have reversed polarity, whereby a pseudopotential Vp(R) due to the radiofrequency field is formed in the ion guide. Here, R denotes a distance from the ion beam axis in a radial direction. The pseudopotential Vp(R) is expressed as follows:Vp(R)={qn2/(4 mΩ2)}·(V/r)2·(R/r)2(n-1) where r denotes the inradius of the ion guide, m denotes the mass of an ion, and q denotes the (electric) charge of the ion. From this expression, the followings can be explained.
When one or more of the inradius r of the ion guide, the amplitude V of the radio-frequency voltage VRF, and the frequency Ω of the radio-frequency voltage VRF is changed, the pseudopotential Vp(R) changes. Therefore, making the pseudopotential Vp(R) change along the ion beam axis forms a gradient of the pseudopotential Vp(R), and force is generated by the gradient, whereby ions can be transported.
However, in the case where ion guides exist continuously along the ion beam axis, as in the case of using rod electrodes, changing the pseudopotential by an electric method, as changing the amplitude V or the frequency Ω of the radio-frequency voltage VRF, is difficult, and the structure of the ion guides, such as the inradius r of the ion guides needs to be changed. However, the structure of the ion guides cannot be changed in use, and thus it is impossible to control ions at will.
Meanwhile, to change the pseudopotential by the electric method such as changing the amplitude V and the frequency Ω, electrodes arranged separately in the direction of the ion beam axis may be used instead of a simple rod electrode, and radio-frequency voltages VRF having different amplitudes V and frequencies Ω may be applied to the respective electrodes. However, in order to control the amplitude V, the frequency Ω, and the like on an analog (sine wave) radio frequency, a complicated device has to be used, which is difficult to be implemented for ion guides.
Thus, in order to apply radio-frequency voltages to electrodes of the ion guide, and to perform control of changing the amplitude of the applied radio-frequency voltage along the ion beam axis, a digital radio-frequency voltage generator that generates rectangular waves is used, in which a direct-current voltage is switched on/off at high speed or positive/negative direct-current voltages are alternately switched at high-speed to generate (pseudo-)radio-frequency voltage. A typical digital radio-frequency voltage generator performs a switching operation in which the positive/negative direct-current voltages are alternately repeated (FIG. 9A). Patent Literature 2 describes that the direct-current voltage is set at 0 for a certain time period when the direct-current voltage is switched from positive to negative, and from negative to positive, that is, the direct-current voltage is switched in the order of 0→+V0→0→−V0→0 during one cycle of a radio frequency (FIG. 9B). Here, by changing the ratio of a time period tv to maintain the direct-current voltage at +V0 and −V0, to a time period t0 to maintain the direct-current voltage at zero, it is possible to change the effective amplitude V of the radio-frequency voltage without changing the cycle and the magnitude of the direct-current voltage.
Switching of voltages illustrated in FIG. 9B is implemented by, for example, an electric circuit 90 illustrated in FIG. 10. A capacitor C in the electric circuit 90 corresponds to an electrode pair in an ion guide. One of the terminals of the capacitor C is grounded, and the other terminal is connected in parallel to a first power supply E1 at a potential +V0 via a first switch S1 and a second power supply E2 at a potential −V0 via a second switch S2. To the two terminals of the capacitor C, an electric resistance R and a zeroth switch S0, which are connected in series to each other, are connected in parallel to the capacitor C.
The switching of voltage in the electric circuit 90 is performed as follows. With the zeroth switch S0 connected in parallel to the capacitor C (electrode pair) being opened, the second switch S2 is opened, and the first switch S1 is closed, whereby a voltage +V0 is applied across the capacitor C (electrode pair) (FIG. 11A). Next, the first switch S1 is opened, and the zeroth switch S0 is closed, whereby the voltage across the capacitor C (electrode pair) becomes zero (FIG. 11B). Subsequently, the zeroth switch S0 is opened, and the second switch S2 is closed, whereby the voltage −V0 is applied across the capacitor C (electrode pair) (FIG. 11C). Furthermore, the second switch S2 is opened, and the zeroth switch S0 is closed, whereby the voltage across the capacitor C (electrode pair) becomes zero (FIG. 11D). Through the above operation, the switching of voltages, 0→+V0→0→−V0→0, that is one cycle of the radio-frequency voltage is performed. Thus, only by changing the ratio of a time period to close the zeroth switch S0 (a time period to maintain the direct-current voltage at zero) t0 to a time period to close the first switch S1 or the second switch S2 (a time period to maintain the direct-current voltage at +V0 or −V0) tv, the effective amplitude V of the radio-frequency voltage can be controlled. This circuit is simpler than that in the case of using an analog (sine wave) radio frequency and can be easily implemented.